Optical transmission systems including optical switching devices, control apparatuses, and methods

ABSTRACT

The optical systems of the present invention include optical switching device generally configured to control signal characteristic profiles over the pluralities of signal channels, or wavelengths, to provide desired signal characteristic profiles at the output ports of the device. Various signal characteristics that can be controlled include power level, cross-talk, optical signal to noise ratio, etc. The optical switching devices can include balanced demultiplexer/multiplexer combinations and switches that provide for uniform optical loss through the devices. In addition, low extinction ratio switches can be configured to provide higher extinction ratios.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part (“CIP”) of, and claimspriority from, commonly assigned U.S. Provisional Application Nos.60/137,835 filed Jun. 7, 1999 and 60/178,221 filed Jan. 26, 2000, whichare incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not Applicable

BACKGROUND OF THE INVENTION

[0003] The present invention is directed generally to opticaltransmission systems. More particularly, the invention relates tooptical switching devices, such as optical cross-connect switches,routers, add/drop multiplexers, and equalizers for use in opticalsystems.

[0004] Digital technology has provided electronic access to vast amountsof information. The increased access has driven demand for faster andhigher capacity electronic information processing equipment (computers)and transmission networks and systems to link the processing equipment.

[0005] In response to this demand, communications service providers haveturned to optical communication systems, which have the capability toprovide substantially larger information transmission capacities thantraditional electrical communication systems. Information can betransported through optical systems in audio, video, data, or othersignal formats analogous to electrical systems. Likewise, opticalsystems can be used in telephone, cable television, LAN, WAN, and MANsystems, as well as other communication systems.

[0006] Early optical transmission systems, known as space divisionmultiplex (SDM) systems, transmitted one information signal using asingle wavelength in separate waveguides, i.e. fiber optic strand. Thetransmission capacity of optical systems was increased by time divisionmultiplexing (TDM) multiple low bit rate, information signals, such asvoice and video signals, into a higher bit rate signal that can betransported on a single optical wavelength. The low bit rate informationcarried by the TDM optical signal is separated from the higher bit ratesignal following transmission through the optical system.

[0007] The continued growth in traditional communications systems andthe emergence of the Internet as a means for accessing data has furtheraccelerated the demand for higher capacity communications networks.Telecommunications service providers, in particular, have looked towavelength division multiplexing (WDM) to further increase the capacityof their existing systems.

[0008] In WDM transmission systems, pluralities of distinct TDM or SDMinformation signals are carried using electromagnetic waves havingdifferent wavelengths in the optical spectrum, i.e., far-UV tofar-infrared. The multiple information carrying wavelengths are combinedinto a multiple wavelength WDM optical signal that is transmitted in asingle waveguide. In this manner, WDM systems can increase thetransmission capacity of existing SDM/TDM systems by a factor equal tothe number of wavelengths used in the WDM system.

[0009] Optical WDM systems are presently deployed as in point-to-pointWDM serial optical links (“PTP-WDM”) interconnected by electrical signalregeneration and switching equipment. At the electrical interconnectionsites in the PTP-WDM systems, all optical signals are converted toelectrical signals for processing. Electrical signals can be droppedand/or added at the site or can be regenerated and retransmitted onnominally the same or a different wavelength along the same fiber pathor switched to a different fiber path.

[0010] As would be expected, it can become extremely expensive toperform optical to electrical to optical conversions in PTP-WDM systemsmerely to pass signals along to the transmission path. The cost ofelectrical regeneration/switching in WDM systems will only continue togrow with WDM systems having increasing numbers of channels andtransmission paths in the system. As such, there is a desire toeliminate unnecessary, and costly, electrical regeneration and switchingof information being transported in optical systems.

[0011] Current optical systems already benefit from the use of limitedcapability, optical switching devices. For example, optical add/dropmultiplexers provide optical access to a single transmission fiber toremove selected channels from a WDM signal. Optical add/dropmultiplexers eliminate the need to convert all of the optical signals inthe transmission fiber to electrical signals, when access to only aportion of the traffic being transmitted as optical signals is required.

[0012] As optical system capacity requirements continue to grow withdemand, it will become increasingly necessary for optical systems toevolve from point to point optical link toward multidimensional opticalnetworks. Numerous optical switching devices have been proposed asalternatives to electrical switching to enable multidimensional alloptical networks. For example, U.S. Pat. Nos. 4,821,255, 5,446,809,5,627,925 disclose various optical switch devices.

[0013] A difficulty with many proposed optical cross-connect switches isthat the switches become overly complex with increasing numbers ofoptical channels and input/output ports. The interconnection of multiplefiber paths, each carrying multiple channels, also becomes extremelydifficult to manage effectively.

[0014] In addition, the interconnection of multiple fiber paths canintroduce additional complications in the system. For example, signalchannels being switched between different fiber paths can have differentpower levels that introduce channel to channel power variations inoptical paths exiting the optical switching devices. The powervariations in optical switching devices are particularly troublesome,because the variations can propagate through numerous optical paths anddegrade the performance throughout a portion of the system.

[0015] In addition, optical switching devices can also imposesignificant optical losses on the signal channels passing through thedevices. As the number of ports, i.e., size, and functionality of theoptical switching devices increases, it generally becomes necessary toamplify the signal channels to overcome splitting losses and otherlosses in the optical switching device. However, optical amplificationto overcome losses associated with optical switching devices can furtherintroduce variations in the signal channels passing through an opticalamplifier.

[0016] Typically, optical amplifiers automatically control thecharacteristics of signal channels passing through the optical amplifierusing either Automatic Power Control (APC) or Automatic Gain Control(AGC) schemes. In APC schemes, the amplifier gain will be variedaccording to the APC scheme to maintain the total output of the opticalsignal exiting the amplifier within a constant power range. As a result,if the number of channels varies, the individual signal channel powerscan vary in APC schemes. In AGC schemes, the amplifier gain ismaintained within a constant gain range. Therefore, if the signalchannel powers or the number of signal channels varies, then the totaloutput power of the optical signal channel will vary as the overall gainof the amplifier is maintained within its constant gain range. As withAPC schemes, the individual signal channel powers can vary in AGCschemes.

[0017] Traditional AGC and APC schemes are typically not effective inoptical switching device configurations. The ineffectiveness is becausesignal channels are often combined from multiple input optical paths,which can have different signal powers. Thus, while the signal channelsin any given input optical path may be controlled, variations in signalpower between the input paths can cause signal channel power variationsin one or more of the output optical paths.

[0018] Signal channel power variations also can be inherently producedas a result of the optical switching device design. The variousprocesses performed in the devices, such as demultiplexing, splitting,switching, adding, dropping, coupling, and multiplexing of varioussignal channels can each introduce variations in the signal channelpower levels.

[0019] For example, in many optical systems and component designs it iscommon to include symmetrically designed demultiplexers andmultiplexers. The symmetrical demux/mux construction provides forstreamlined manufacturing of the products and the potential forbi-directional use in bi-directional systems. However, symmetricaldemux/mux configurations can produce signal channel power variations, ifdeployed with all optical switching devices.

[0020] Analogously, dissimilar demux/mux configurations are oftendeployed in various optical switching devices. For example, wavelengthselective demultiplexers can be used with non-wavelength selectivecombiners, such as N:1 couplers. These configurations can also introducesignal channel power variations.

[0021] Depending upon the number of channels in the WDM system, one ormore stages of non-wavelength selective splitting and combining can beused along with various filtering techniques applied to each wavelength.For example, see U.S. Pat. No. 5,446,809 (the “'809 patent”), which isincorporated herein by reference. Unfortunately, as the number ofchannels in WDM systems continues to increase, non-wavelength selectivesplitting and coupling can become impractical even when opticalamplification is used to overcome the passive losses.

[0022] Another source of signal channel degradation in optical switchingdevices occurs when the switches incompletely block signal channels,thereby allowing leakage of unwanted signal channels through the device.Leakage of unwanted signal channels will degrade the signal quality ofthe signal channels being passed through the devices by creatingcross-talk interference. Significant levels of cross-talk can destroythe information carried by signal channels being passed through thedevice. The amount of crosstalk that occurs through an on/off switch canbe characterized by the extinction ratio of the switch, which is theratio of power transmitted through the switch in the on state over theoff state.

[0023] There are numerous types of optical switches, or gates, such asmechanical, thermo-, acousto-, and electro-optic, doped fiber andsemiconductor gates, and tunable filters, that are available for use inoptical switching devices. Thermo-, acousto-, and electro-optic switchesare appealing, because of the relatively low cost and solid statecharacteristics. However, these switches often have extinction ratiosonly on order of ˜20 dB. The low extinction ratios can introduceunacceptable levels of cross-talk in optical systems. As such, thesetypes of switches are typically limited to use in systems in which minorsignal degradation can be tolerated or the degradation does notaccumulate from multiple switches.

[0024] Conversely, mechanical line or mirror switches can have excellentextinction ratios, but the moving parts associated with mechanicallymoving and aligning the switch can pose a reliability problem over thelong term. Doped fiber and semiconductor on/off gates can provide goodextinction, but require active components to maintain the gateconfigurations, which increases cost and decreases reliability. Tunablefilters can also provide good extinction ratio in a switch mode, but thefilters may require active components that have maintainable, stabletuning characteristics.

[0025] The various signal degradation mechanisms that can be introducedin an optical system by prior art optical switching devices have been acontributing factor to the industry's inability to develop and deployreliable optical switching devices to enable the creation ofall-optical, or “transparent”, networks. Accordingly, there is a needfor optical systems including optical amplifiers and optical switchingdevices that provide increased control over signal channels beingswitched, or routed, through the system.

[0026] As the need for high capacity WDM systems continues to grow, itwill become increasingly necessary to provide all optical networks thateliminate the need for and expensive of electrical regeneration toperform signal routing and grooming in the networks. The development ofmulti-dimensional, all-optical networks will provide the cost andperformance characteristics required to further the development of highcapacity, more versatile, longer distance communication systems.

BRIEF SUMMARY OF THE INVENTION

[0027] The apparatuses and methods of the present invention address theabove need for higher performance optical systems. Optical systems ofthe present invention generally include at least one optical processingdevice, such as optical cross-connect switches and routers, as well asadd/drop multiplexers, disposed along an optical path betweentransmitting and receiving optical processing nodes.

[0028] Optical signals from the transmitting nodes pass through theoptical switching devices via input and output ports to the receivingnodes. In various embodiments, the optical switching device includesoptical amplifiers, such as EDFAs, configured to provide automatic gain& power control (AGPC) over the output power of signal channels passingthrough the optical amplifier. AGPC allows signal channels from diverseoptical paths to be combined without incurring a substantial performancepenalty due to non-uniform signal power levels in the combined signalchannels.

[0029] The optical switching devices generally are configured to controlthe signal characteristic profile over the pluralities of signalchannels, or wavelengths, and provide a desired signal characteristicprofile at the output ports of the switching devices. Various signalcharacteristics controlled in the devices include power level,cross-talk, optical signal to noise ratio, etc. The optical switchingdevices can include balanced demultiplexer/multiplexer combinations andswitches that provide for uniform optical loss and provide for variablepower control of the signal channels passing through the devices. Inaddition, low extinction ratio switches can be configured to providehigher extinction ratio switching devices.

[0030] Accordingly, the present invention addresses the aforementionedneeds by providing optical systems, apparatuses, and methods withincreased control over signal channels being combined from differentoptical paths. These improvements provide for the use of opticalswitching devices in optical systems without incurring a substantialdecrease in the performance of the system. These advantages and otherswill become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Embodiments of the present invention will now be described, byway of example only, with reference to the accompanying schematicdrawings for the purpose of illustrating embodiments only and not forpurposes of limiting the same, wherein:

[0032]FIGS. 1-2c illustrate exemplary optical systems includingexemplary optical switching devices;

[0033]FIG. 3 illustrates an exemplary optical switching device;

[0034]FIGS. 4a-4 c illustrate exemplary optical amplifierconfigurations;

[0035]FIGS. 5a-6 c illustrate exemplary optical switching devices;

[0036]FIGS. 7a-9 b illustrate exemplary demultiplexer and multiplexerconfigurations;

[0037]FIGS. 10a-10 b illustrate exemplary optical switching devices;

[0038]FIGS. 11a-11 c illustrate exemplary multiple filter packages; and,

[0039]FIGS. 12-14 illustrate exemplary optical switch elements.

DESCRIPTION OF THE INVENTION

[0040] Optical systems 10 of present invention include at least oneoptical switching device 12 interconnecting two or more other opticalprocessing nodes 14. Various guided or unguided transmission media, suchas optical fiber 15, can provide an optical link 16 allowing for opticalcommunication between the nodes 12. The optical switching devices 12generally include optical routers, optical cross-connect and lineswitches, add/drop devices, and other devices interconnecting opticalfibers that provide the optical links 16. In the context of the presentinvention, an optical switching device 12 receives an optical signal atan input port and can pass or substantially block the passage of one ormore optical signal wavelengths, or signal channels, λ_(i) included inthe optical signal to an output port, without converting the opticalsignal to electrical form.

[0041] The system 10 can be configured in various linear andmulti-dimensional architectures (FIG. 1) and controlled by a networkmanagement system 18. One or more optical amplifiers 20 can be disposedperiodically along the optical links 16, e.g., every 20-120 km, toamplify attenuated optical signals during transmission between nodes 14and/or proximate to other optical components to provide gain to overcomecomponent losses. One or more optical fibers can be disposed to providemultiple optical link 16 between nodes 14 along a common optical path.In addition, each fiber can carry uni- or bi-directionally propagatingoptical signals depending upon the system 10 configuration.

[0042] Generally, the optical processing nodes 14 can include one ormore optical transmitters 22 and optical receivers 24, as well asoptical switching devices 12. Various combinations of optical switchingdevices 12, transmitters 22, and receivers 24 can be included in eachprocessing node 14 depending upon the desired functionality in the nodes14.

[0043] The optical transmitters 22 and optical receivers 24 areconfigured respectively to transmit and receive optical signalsincluding one or more information carrying optical signal wavelengths,or signal channels, λ_(i). In the present description, the term“information” should be broadly construed to include any type ofinformation that can be optically transmitted including voice, video,data, instructions, etc.

[0044] The transmitters 22 used in the system 10 generally will includea narrow bandwidth laser optical source, such as a DFB laser, thatprovides an optical carrier. The transmitters 22 also can include othercoherent narrow or broad band sources, such as sliced spectrum or fiberlaser sources, as well as suitable incoherent optical sources asappropriate. Information can be imparted to the optical carrier eitherby directly modulating the optical source or by externally modulatingthe optical carrier emitted by the source. Alternatively, theinformation can be imparted to an electrical carrier that can beupconverted onto an optical wavelength to produce the optical signal.The information can be amplitude, frequency, and/or phase modulatedusing various formats, such as return to zero (“RZ”) or non-return tozero (“NRZ”) and encoding techniques, such as forward error correction(“FEC”).

[0045] The optical receiver 24 used in the present invention will beconfigured to correspond to the particular modulation format used in thetransmitters 22. The receiver 24 can receive the signals using variousdetection techniques, such as coherent detection, optical filtering anddirect detection, and combinations thereof. Employing tunabletransmitters 22 and receivers 24 in the optical nodes 14 in a network,such as in FIGS. 1-3, can provide additional versatility in configuringthe network architecture of the system 10.

[0046] The transmitters 22 and receivers 24 can be also connected tointerfacial devices 25, such as electrical and optical cross-connectswitches, IP routers, etc., to provide interface flexibility within, andat the periphery of, the optical system 10. The interfacial devices 25can be configured to receive, convert, and provide information in one ormore various protocols, encoding schemes, and bit rates to thetransmitters 22, and perform the converse function for the receivers 24.The interfacial devices 25 also can be used to provide protectionswitching in various nodes 14 depending upon the configuration.

[0047] Optical combiners 26 can be used to combine the multiple signalchannels into WDM optical signals. Likewise, optical distributors 28 canbe provided to distribute the optical signal to the receivers 24 _(j).The optical combiners 26 and distributors 28 can include variousmulti-port devices, such as wavelength selective and non-selective(“passive”), fiber and free space devices, as well as polarizationsensitive devices. For example, circulators, passive, WDM, andpolarization couplers/splitters, dichroic devices, prisms, diffractiongratings, arrayed waveguides, etc. can be used alone or in variouscombinations along with various tunable or fixed wavelength transmissiveor reflective, narrow or broad band filters, such as Bragg gratings,Mach-Zehnder, Fabry-Perot and dichroic filters, etc. in the opticalcombiners 26 and distributors 28. Furthermore, the combiners 26 anddistributors 28 can include one or more stages incorporating variousmulti-port device and filter combinations to multiplex, demultiplex,and/or broadcast signal channels λ_(i) in the optical systems 10.

[0048] As shown FIG. 2a, the optical switching device 12 can be embodiedas an optical router, or cross-connect switch, including a plurality ofinput ports 30 _(I) and output ports 30 _(O). In FIG. 2a, the device 12,transmitters 22 and receivers 24 are shown as separate nodes forconvenience. It will be appreciated that devices 12, the transmitters 22and receivers 24 can be located in different or the same physical node14 depending upon the system configuration. The device 12 is shown witheach of the input ports 30 _(I) and output ports 30 _(O) opticallyconnected via an optical switch path 32 and a switch element 34 disposedalong one of the optical switch path 32. It will be appreciated thatdevice 12 can include multiple optical switch paths between input andoutput ports and that some input and output ports may not be opticallyconnected.

[0049] The switch element 34 will generally be disposed along eachoptical switch path 32 connecting the input and output ports to allowselective passage or blockage of the optical signals in the opticalswitch paths 32. The switch element 34 can include wavelength selectiveor non-selective on/off gate switch elements, as well as variableoptical attenuators configured to have a suitable extinction ratio aswill be further discussed. The switch elements 34 can include signalpath, as well as multiple path elements employing polarization,interferometry, or other effects to perform the gating and/or variableattenuation function. Wavelength selective switch elements 34 _(s) canbe used with various types of distributors 28 and combiners 26. Whereas,non-selective switch elements 34 _(ns) will generally require some levelof demultiplexing in the distributors 28, unless only optical lineswitching is performed.

[0050] The optical switching device 12 can be configured to selectivelyswitch and route signal groups that may include individual signalchannels and/or groups of signal channels including up to all of thesignal channels in an optical signal λ_(I,1-N) or line switch theoptical signal provided to the input port 30 _(I) to one or more of theoutput ports 30 _(O). Optical switching devices 12, such ascross-connects, routers, and add/drop multiplexers, that can be used toswitch and route groups of signal wavelengths, or signal channels, aredescribed in PCT Publication No. WO/?, which is incorporated herein byreference.

[0051] The devices 12 can be configured to perform various otherfunctions, such as signal group filtering, power equalizing, addingtelemetry and markers to the signal channels, etc., in the system 10. Invarious embodiments, the switch elements 34 can include binary on/offgates in combination with variable attenuators or variably attenuatingon/off gates. Variable attenuation in combination with opticalamplification and on/off switching in the switch elements 34 provide forchannel filtering and power equalization, or gain trimming, of thesignal group power passing through the device 12. In variousembodiments, variable optical attenuators, such as those commerciallyavailable from JDS-Uniphase, Inc., can be used to variably control thesignal group output power from the switch element 34 to provide for gainequalization with binary on/off gates. Alternatively, optical amplifiers20 can be configured to operate as variably attenuating on/off gates, aswill be further described.

[0052] As shown in FIG. 2b, the devices 12 can be deployed in 1×1, inaddition to N×M, configurations along the optical link 16 to providesignal group/channel filtering and equalization. In 1×1 configurations,the distributor 28 can demultiplex optical signals into respectivesignal groups that can be separately filtered or equalized by the switchelements 34 before being recombined into an output signal, as will befurther described.

[0053] In addition, distributors 28 and combiners 26, typically passivesplitters and couplers, respectively, can be provided on the input andoutput to provide drop access for receiving the channels from the fiber16 and/or to add channels to the fiber 16. The switch elements 34 in thedevice 12 can be configured to substantially block or pass thosechannels that were received from the drop access depending upon whetherwavelengths are being reused or multicast. Similarly, the switchelements 34 can be configured to block wavelengths including the samesignal channels as those being added to the system 10 after the device12. It will be appreciated that wavelength selective add and/or dropmultiplexers also can be used along with the device 12 or the device 12can be configured to provide dedicated add/drop channels. However, somewavelength selective add/drop embodiments can limit the flexibility inreconfiguring the system 10.

[0054] Alternatively, the device can be configured in 2×1, 2×2 (FIG.2c), and 1×2 configurations to accommodate adding and/or dropping ofsignal channels using the device 12. Optical transmitters 22 and/orreceivers 24 can be respectively coupled to the input port 30 _(I) andoutput port 30 _(O) to selectively add and/or drop signal channels usingthe device 12. The distributors 28 can include various combinations ofpassive splitters, filters, and demultiplexers to provide the inputsignals to the switch elements 34, as will be further described.

[0055] In the various add/drop embodiments of the devices 12, the dropreceivers 24 and add transmitters 22 can be locally or remotely locatedfrom the switching device 12. In various embodiments, add and drop portsof the device 12 can be directly connected with a local system, such asa ring, as shown in FIG. 1. In addition, tunable transmitters 22 andreceivers 24 are particularly advantageous in add/drop configurations ofthe device 12. In these embodiments, the device 12 can be dynamicallyreconfigured along with the transmitter and receiver wavelengths toreconfigure the signal channel plan of the system 10.

[0056] OA BP

[0057] Optical switching devices 12, particularly those having outputports 30 _(O) provided with signal channels from multiple input ports 30_(I), can significantly impact the performance of the system 10. Forexample, variations in signal channel power in the combined opticalsignals can affect the performance of optical amplifiers 20 andreceivers 24 disposed along the optical link 16 connected to the outputports 30 _(O). Therefore, it is important to control the signal channelpower levels λ_(i) not only within each optical signal provided todevice 12, but across all optical signals that pass through the device12 to output fibers 30 _(O).

[0058] The devices 12 may include one or more amplifiers 20 in variousconfigurations relative to the input and output port 30 to overcomeoptical losses associated with the optical switching devices 12, inaddition to along the optical link 16. For example, it may be necessaryto amplify the signal channels to overcome splitting losses in thedevices 12 having multiple output ports 30 _(I) to which the signalchannels may be multicasted.

[0059] As shown in FIG. 3, the optical amplifiers 20 can be locatedbefore or after the distributor 28 or switch elements 34. The opticalamplifiers 20 can also be incorporated as the switch elements 34 toprovide consolidated amplification, on/off switching, and variableattenuation of signals passing the device 12. For example, in the “off”state/position, an unpumped erbium doped fiber amplifier will absorboptical energy, thereby blocking signal channels λ_(i) from passingthrough the switch element 34 to the output port 30 _(O). Whereas, inthe “on” state/position, an optically pumped EDFA will amplify thesignal channels λ_(i), thereby serving as an amplifier and on/off gatepassing the signal channels λ_(i) to the output port. Furthermore, thepump supplied to the optical amplifier can be variably controlled to notonly turn the gate on, but to control the output power of the signalgroup from the switch element. The consolidated functionality of theoptical amplifiers 20 as switch elements 34 provides for signal channelpower equalization and channel filtering in 1×1 and NxM configurationsof the device 12. In addition, telemetry and device identifierinformation, etc. can be received and/or imparted to the signal channelspassing through the device 12 by imparting information onto the pumppower being supplied to the amplifier 20.

[0060] Optical amplifiers 20 in the device 12 can be disposed betweenthe input and output ports and configured to provide automatic gain &power control (AGPC) over the output power of signal channels λ_(i)passing through the optical amplifier 20. By controlling the signalchannel output power and the loss in the device 12, the performance ofthe overall system 10 can be maintained, when signal channels frommultiple paths are combined using the device 12.

[0061] In various embodiments, the gain of the amplifier 20 iscontrolled based a gain set point G_(s), which itself can be fixed orcontrolled based on the number of signal channels in the optical signaland the input power of the signal channels. As generally shown in FIG.4a, optical signal distributors 28, such as optical taps, can providedbefore and/or after an amplifying medium 36 to monitor the input andoutput signal channel powers. The distributors 28 are opticallyconnected to respective optical to electrical converters 38 that can beconfigured to provide electrical signals corresponding to the signalchannel input and output powers and the number of signal channels.

[0062] The amplifier gain can be calculated from the input and outputpower using a divider circuit 40, which can then be compared to the gainset point G_(s) for the optical amplifier 20 using a comparator circuit42. The output of the comparator circuit 42 is used to control the powersupplied to the optical amplifier 20 by a power source 44, typically bycontrolling the drive current to the power source 44. A centralprocessor 46 can be used to reset the gain set point G_(s) based on theamplifier input power and the number of signal channels passing throughthe amplifier 20.

[0063] Alternatively, the amplifier gain performance can be calibratedas a function of the power supplied by the power source 44 and the inputpower and the number of signal channels. The overall input power wouldbe detected and compared in the comparator 42 to an input power setpoint P_(s). The output of the comparator circuit 42 would again be usedto control power provided by the power source 44. The central processor46 calculates the number of signal channels passing through theamplifier based on an expected range of per channel power levels andresets the input power set point P_(s) to accommodate variations in thenumber of signal channels. Similar configurations can be provided basedon monitoring the signal power at the amplifier output. While theamplifier control loops generally have been described in an analogfashion; digital implementations of the control loop can also beemployed.

[0064]FIG. 4b illustrates a more specific exemplary embodiment of theoptical amplifier 20 including an amplifying fiber 50, such as an erbiumdoped fiber amplifier, supplied with optical energy from pump sources52. The pump sources 52 can provide optical energy, “pump power” in oneor more pump wavelength λ_(pi), which are counter-propagated and/orco-propagated with signal channels λ_(i). For example, pump power in the980 nm and/or 1480 nm wavelength ranges can be provided to the erbiumdoped fiber 50 to amplify signal channels in the 1550 nm range.

[0065] Passive couplers 54, such as high ratio optical taps, can be usedto split a percentage of the optical signal to photodiodes 56 provide anelectrical signal corresponding to the optical signal power to thecentral processor 46. Signal channel counting devices 58 can be used atthe input and/or output of the amplifier 20 to determine the number ofsignal channels passing through the amplifier 20. The central processor46 controls the gain and power set points, G_(s) and P_(s), based on thenumber of channels and the optical signal power. The central processors46 also can compare the channel information from the signal countingdevices 58 with channel information from the network management system18 to verify system performance and take appropriate actions, ifdiscrepancies occur.

[0066] Signal channel counting devices 58 can be embodied as one or morephotodiodes 56 with fixed or tunable filters or as optical spectrumanalyzers configured to count the number of signal channels. Thecounting devices 58 also can be configured to provide the individualsignal channel power levels. Alternatively, the number of signalchannels also can be transmitted along with the signal channels. Inthese embodiments, the counting devices 58 also can include an opticalor electrical filter to separate the signal channel number identifiersignal. For example, the number of signal channels could be transmittedon a low frequency tone to allow it to be detected separately from thesignal channels and with lower cost electronics. A single tone could beused to indicate the number of channels, or each channel could have itsown channel identifier tone. In addition, the number of signal channelscan be transmitted on a separate wavelength or in the overheadinformation carried by the signal channels. Channel identificationinformation also can be included along with the system supervisoryinformation that is transmitted to each network element, i.e.,amplifiers 20, nodes 14, etc. and/or calculated from the total power.

[0067] In accordance with present invention, if the number of signalchannels passing through the amplifier 20 increases or decreases, theAGPC loop will initially act to maintain the gain of the amplifier 20 atthe gain set point G_(s). Contemporaneously, the central processor 46will detect a change in the number of signal channels and the inputpower and adjust the gain set point G_(s) to maintain the desired outputpower for the signal channels exiting the amplifier 20. Additionally, ifthe signal channel input powers vary and the number of signal channelsremains constant, the central processor 46 will adjust the gain setpoint to maintain the desired signal channel output power. Thus, signalchannels from diverse optical paths can be combined with minimalperformance penalty due to non-uniformly amplified signal channels.

[0068] Furthermore, when channel identifier tones are used, theamplifier control scheme can be synchronized to minimize unnecessarychanges to the amplifier set points. For example, the number of channelspassing through the amplifier can be verified as described before thegain or power is varied in response to detected power variations.

[0069] Non-uniformities also can be introduced across the signal channelλ_(i) range, because of varying optical losses that can occur in thedistributors 28, combiners 26, and switch elements 34 used in thedevices 12. The variations are often introduced because the wavelengthselective devices used to separate WDM signals generally exhibitdifferent performance in terms of extinction ratios, loss, and otherproperties between transmission and reflection. As such, prior devicescan introduce signal channel power variations that can degrade systemperformance.

[0070] In the present invention, optical switching devices 12 includesdemultiplexers 60, multiplexer 62, and passive splitter 64 and couplers66 that are configured in various combinations along with the switchelements 34 to balance the optical loss over the signal channel λ_(i)range. The distributors 28 and combiners 26 can be deployed as one ormore stages in various configurations depending upon the desired device12 characteristics including whether wavelength selective switchelements 34 _(s) or non-selective (“non-selective”) switch elements 34_(ns) are used. FIGS. 5a-5 c depict exemplary embodiments of the devices12.

[0071] In FIG. 5a embodiments, passive splitters 64 are provided at theinput ports 30 _(Ii) of the device 12 to passively split the inputsignals λ_(Ii,1-Ni) and provide the entire input signal to each outputport, or a subset thereof, via the associated selective switch element34 _(s) and combiners 26. Passive couplers 66 can be provided at theoutput ports 30 _(oi) to combine the signals passing through selectiveswitch elements 34 _(s). Output signals λ_(oi,1-mi) are provided at eachoutput port 30 _(oi) for further transmission along the optical path orprovided to receiver 24 collocated with the device 12 at combinedswitching and destination nodes 14.

[0072] Wavelength selective filters, such as in the '809 patent, can beused to selectively pass or substantially prevent the passage of theindividual wavelengths in the input signal. Alternatively, ademultiplexer 60 and multiplexer 62 in combination with non-selectiveswitch elements 34 _(ns) can be used to selectively pass orsubstantially prevent the passage of signal groups as shown in FIG. 5b.

[0073] Conversely, in FIG. 6a embodiments of the device 12,demultiplexers 60 can be provided to demultiplex the input signalsreceived from the input ports 30 _(Ii) into a plurality of signalgroups. The signal groups can be split using passive splitters 64 andprovided via switch elements 34 and couplers 66 to respective outputport 30 _(o). The switch elements 34 in these embodiments can benon-selective switch elements 34 _(ns) or wavelength selective switchelements 34 _(s). Additional demultiplexing can be performed after thesplitter 64 to further separate the signal groups, if so desired.

[0074] Various other combinations of splitters 64 and demultiplexers 60and couplers 66 and multiplexers 62 are possible in the presentinvention. For example, in FIG. 6b embodiments, one or more splitterstages 64 can be used to provide replicate signals corresponding to thenumber of signal groups. The replicate signals can be split further andprovided to selected, or all, output ports of the device 12. In FIG. 6bembodiments, additional amplification may be necessary to overcome themultiple splitting losses in the device 12. In these embodiments, thesignal channels can be double passed through one optical amplifier 20using optical circulators 68 to eliminate the need for separateamplifiers before and after the switch elements, as shown in FIG. 6c.Other multi-port device configurations can be used in lieu of, orcombination with the circulator 68 in FIG. 6c embodiments. Also, asignal group filter 70 can be included in the double configuration toincrease the rejection of unwanted signal groups.

[0075] The device 12 can be configured to provide one input port to oneoutput port, multicast, and broadcast switching capabilities. Forexample, 1×N line switches 65 can be used in lieu of, or in combinationwith, splitter 64 and switch element 34 embodiments, as shown in FIG.6d. The line switch 65 is used to selectively direct the entiredemultiplexed signal group to only one of the output ports of the device12. In addition, it is not necessary that each input port 30 _(i) beconnected to each output port 30 _(o) or that only one type of switchelement 34 be used in the device 12. In some instance, it may bedesirable to provide connectivity between only a subset of the input 30_(i) and output ports 30 _(o) or to provide additional ports for futuresystem upgrades.

[0076] The distributors 28, switch elements 34, and combiners 26 areselected to provide a substantially uniform optical performance over thesignal channel range. When non-selective switch elements 34 _(ns) areused in the device 12, the optical signals provided to the input port 30_(I) can be demultiplexed into signal groups of a desired opticalswitching granularity before switching. The non-selective switch element34 _(ns) will generally provide a substantially uniform loss over thesignal channel range because it either passes or blocks the entireoptical signal. Therefore, it is generally necessary to balance theoptical loss of the demultiplexer and multiplexer (“demux/mux”) over thesignal channel range.

[0077] In various embodiments, the optical amplifiers 20, associatedwith the devices 12, as well as variable optical attenuators, can beconfigured to compensate for non-uniform loss within the device 12. Inthis manner, the demultiplexer, multiplexer, and switch elements 34 canbe optimized for other signal characteristics and the optical amplifiers20 will balance the overall loss through the device 12. The opticalamplifiers 20 can be variously located proximate the demux/mux andswitch elements, as described with respect to FIG. 3.

[0078] The demux/mux loss can generally be balanced by exposing eachwavelength to the same number of band-pass (transmission) filters, aswell as the same number of band-stop (reflection) filters; and, thus,the same plural total number of filter stages. The band-pass andband-stop filters can include appropriately configured high-pass orlow-pass filters. In addition, it is not necessary that number ofpass-band stages be equal to the number of stop-band stages.

[0079]FIGS. 7-10 show exemplary demultiplexers and multiplexers thatprovide for a substantially uniform optical loss across the signalchannel range. While FIGS. 7-10 show Bragg gratings as exemplaryfilters, other filters including those described herein can besubstituted as appropriate. Also, the various signal group filters canbe the same or different design and can used to filter signal groupsthat include different number of signal channels.

[0080]FIGS. 7a and 7 b show linear demultiplexers 60 and multiplexers62, respectively, for use in the present invention. As shown in FIGS. 7aand 7 b, each signal groups 1-N is selectively filtered using a signalgroup filters 70, such as a Bragg grating 72 and removed via circulator68. The signal group filters 70 used in the linear configurations areshown as Bragg gratings 72; however, Fabry-Perot filters also can used,as well as WDM couplers, etc., as previously described.

[0081] Similarly, FIGS. 8a and 9 a and 8 b and 9 b show paralleldemultiplexers 60 and multiplexers 62, respectively, for differingnumbers of signal channels or groups of signal channels. The separationof the signal groups in these configurations is performed in parallel,as opposed to serially. While parallel designs are generally moreefficient than linear designs, multiple signal group processing inparallel stages can introduce signal cross-talk.

[0082]FIGS. 10a and 10 b show single stage demultiplexers 60 andmultiplexers 62 in combination with wavelength selective switchingelements 34. Wavelength controllers 73 are provided to communicate withthe central processor 46 and control the wavelength of the filters 70/72in accordance with switch configurations provided by the networkmanagement system 18.

[0083] In FIGS. 10a and 10 b configurations, both the demux/mux stagesare balanced, as well as the switch element 34 stages. Conversely, invarious embodiments, wavelength selective switch element stages and thedemux/mux stages provide overall balance of the optical switching device12; however, neither the switch element 34 stages nor the demux/muxstages are separately balanced.

[0084] It will be appreciated that wavelengths can be selectivelyswitched in various orders depending upon the system configurations. Forexample, alternating the wavelengths or wavelength groups switched ineach stages can increase the rejection of adjacent wavelengths.Likewise, switching multiple wavelengths or wavelength groups in eachstage can be used to minimize the overall loss through the switchingelements. In addition, the switching and/or demux configurations can bevaried depending upon the wavelengths used in the optical system and thetype of wavelength selective devices used. For example, seriallyconnected Bragg gratings can be arranged to reduce cladding modereflections of shorter wavelengths by longer wavelength gratings thatcan result in crosstalk interference as will be further discussed.

[0085] Bragg grating filters 72 can be reliable manufacture to provideboth very narrow (<0.1 nm) and extremely broad wavelength filters (>20nm) for use in the present invention. Also, the reflective wavelength ofBragg grating filters 72, as well as other filters, often has atemperature dependence that allows the filters 70 to be thermally tuned.However, the temperature dependence also can cause the filterwavelengths to shift undesirably during operation. As such, it isgenerally necessary to control the temperature of the grating or packagethe grating such that the thermally variations are offset. Bragg gratingpackaging generally relies on the additional dependence of Bragggratings on strain to offset the temperature variations, such asdescribed in U.S. Pat. No. 5,042,898.

[0086] In demultiplexers 62 and switch elements 34, it may be necessaryto control the temperature or tune numerous signal group filters 70within the device 12. In various embodiments, wavelength controllers 73provide temperature control over multiple signal group filters 70, suchas Bragg gratings 72, using a common cooling element 74, such as athermo-electric cooler, and individually heating elements 76. Eachfilter 70 is surrounded by high thermal conductivity material 78, suchas metallic compounds, to ensure good thermal contact with the heatingand cooling elements.

[0087] Conversely, adjacent filters 70 are separated by low thermalconductivity material 80, such as polymers, etc. to thermally isolateeach filter 70, thereby allowing individual temperature control via theheating elements 76. One or more temperature sensors 82, such asthermocouples and thermistors, can be provided adjacent to the filters70 to monitor the temperature and provide for feedback control of theheating elements 76 and/or cooling elements 74. FIGS. 11a and 11 billustrate cross-sectional, side views of two parallel filter packages.FIG. 11a embodiments provide for redundant cooling elements on oppositesides of the filter 70. Whereas, in FIG. 11b embodiment, one directionalheat transfer via the high thermal conductivity material 78 is providedto the cooling element 74, which also can be deployed in a redundantconfiguration.

[0088] Various non-selective switch elements 34 _(ns) can be used inpresent invention, such as mechanical line and micro-mirror (“MEM”)switches, liquid crystal, magneto-optic, thermo-optic, acousto-optic,electro-optic, semiconductor amplifier, etc., as well as erbium dopedfiber amplifier switch elements previously described. Alternatively, theswitch elements 34 can employ wavelength selective multi-port devicesand filters, such as WDM couplers, circulators, Bragg gratings,Mach-Zehnder, Fabry-Perot and dichroic filters, as shown in theexemplary embodiments of FIG. 10. As previously described, the switchelement 34 can be configured strictly as on/off gates and/or variableattenuating on/off gates. For example, the nonselective switch 65 shownin FIG. 6b can be embodied as any of the foregoing switch types, and caninclude an optical attenuator to control the signal power.

[0089] It is generally desirable to use a switch element 34 that has ahigh extinction ratio, e.g., >40 dB, to substantially prevent thepassage of a signal through the switch in the off position. Signalleakage through switches in the off position can severely degradesignals passing through the device 12 by causing crosstalk interferencebetween the signals. Lower extinction ratio, <40 dB, switch elements 34that significantly attenuate can be deployed in various configurationsdepending upon the system configuration.

[0090] The extinction ratio required to prevent significant systemperformance degradation from cross-talk interference depends, in part,on the system configuration. For example, lower extinction ratio switchelements 34 may be sufficiently effective to prevent significantcrosstalk degradation of the signals, when there are no other sources ofsignificant crosstalk in the system. Conversely, when multiple devices12 or other potential sources of crosstalk, such as mux/demuxconfigurations, are concatenated, the extinction ratio of the devices 12generally has to be higher to prevent an unacceptable cumulativecrosstalk degradation of the system performance.

[0091] In the present invention, low extinction ratio switch elements,such as a thermo-optic and electro-optic switches, as well as wavelengthselective switches with low extinction ratios, e.g., <99% reflective,can be configured to provide higher extinction ratio devices. As shownin FIG. 12, the switch element 34 includes a low extinction ratio on/offswitch element 34 ₁, such as a variable optical attenuator, that isserially connected to a saturable absorber 84. The saturable absorber 84is designed to absorb low levels of optical energy characteristic ofoptical signal leakage, when non-selective switch element 34, is in theoff configuration. However, the saturable absorber 84 is furtherdesigned to saturate at higher levels of optical energy, therebyallowing the remaining optical energy in the optical signal to passthrough the switch element 34. Suitable saturable absorbers for use inthe present invention include erbium doped fiber, semiconductoramplifiers, or other saturable media.

[0092] Lower extinction ratio switch elements 34 also can be embodied as2×2 crossbar switch having an input port (In) and an output port (Out)alternately connected to first and second input/output ports, In/Out,and In/Out₂, respectively. The first and second input/output ports areoptically connected via an optical fiber path 16. An optical isolator 86is provided in the path 16 to prevent optical energy from passing fromthe second input/output port to the first input/output port. In a first(“on”) switch configuration shown in FIG. 13a, the optical signal passesfrom the input port through the first crossbar 88 ₁, the first andsecond input/output port, the second crossbar 88 ₂, and exits the switchelement 34 via the output port. In a second (“off”) configuration, shownin FIG. 13b, the optical signal passes from the input port through thefirst crossbar 88 ₁ and the second-input/output port and is preventedfrom reaching the first input/output port by the isolator 86. Any signalleakage from the input port to the first input/output port will befurther attenuated by the leakage path, shown as dashed lines, from thesecond input/output port to the output port of the switch 34.

[0093] Various switching elements, such as liquid crystal switches, canhave polarization dependent extinction ratios that can introduce varyingamounts of crosstalk into the system 10. FIG. 14 illustrates embodimentsof the present invention to decrease the polarization dependence ofthose switch elements 34. Signals passing through the switch element 34are reflected by a Faraday rotator 90, which also rotates thepolarization of the signals. The polarization rotated signal passthrough the switch element 34 a second time, so that each portion of thesignal has passed through the switch element in both polarizations. Inthese embodiments, not only can the polarization dependence of theoverall be eliminated, but the device has an effective extinction ratiothat is higher than a single pass through the switch element 34.

[0094] In the operation of the present invention, input signalsincluding one or more signal channels are provided to the device 12 atvarious input ports from various transmitters in the system 10. Thedevice 12 is configured to selectively switch the signal channels insignal groups including one or more signal channels to output portsleading to receivers at the signal destination. The device 12 balancesthe optical signal characteristics through the device, such that signalchannels combined from various input ports have substantially equalsignal characteristics including signal power, cross-talk noise, etc.

[0095] Those of ordinary skill in the art will appreciate that numerousmodifications and variations can be made to specific aspects of thepresent invention without departing from the scope of the presentinvention. It is intended that the foregoing specification and thefollowing claims cover such modifications and variations.

1. An optical transmission system comprising: at least one opticaltransmitter configured to transmit at least one optical signal channelvia an optical signal along an optical path; at least one opticalreceiver configured to receive at least one optical signal channel fromsaid optical path; and, an optical switching device disposed along saidoptical path including: a demultiplexer configured to demultiplex theoptical signal provided to an input port into a plurality of signalgroups, each signal group including at least one signal channel, aswitch element configurable to pass or prevent the passage of at leastone of the signal groups and variably control the optical power of thesignal groups passing though said switch element, and a multiplexerconfigured to multiplex the signal groups passed by said switch elementinto an output signal through an output port in optical communicationwith said optical receiver, wherein said optical switching deviceimparts a substantially uniform optical loss across the at least oneoptical signal channels; a distributor connected along the optical pathat an input port of the demultiplexer; at least one drop receiverconnected to an output port of the distributor; a combiner connectedalong the optical oath at an output port of the multiplexer; and atleast one add transmitter connect to an input port of the combiner. 2-24(cancelled).
 25. The optical transmission system of claim 1, wherein theswitch element is configured to block wavelengths that are beingdropped.
 26. The optical transmission system of claim 25, wherein theswitch element is configured to block a signal group comprising aplurality of wavelengths, wherein at least one of the wavelengths in thesignal group is being dropped.
 27. The optical transmission system ofclaim 25, wherein the switch element is configured to block wavelengthsthat are being added.
 28. The optical transmission system of claim 1,wherein the distributor is wavelength-selective and provides to thereceiver only signals being dropped.
 29. The optical transmission systemof claim 1, wherein the at least one add transmitter and the at leastone drop receiver are remotely located from the switching device. 30.The optical transmission system of claim 1, wherein the at least onedrop receiver and the at least one add transmitter are connected to alocal optical transmission system.
 31. The optical transmission systemof claim 30, wherein: at least one drop receiver is located in the localoptical transmission system remote from the switching device; and atleast one add transmitter is located in the local optical transmissionsystem remote from the switching device.
 32. The optical transmissionsystem of claim 1, wherein: the at least one drop receiver is a tunablereceiver; and the at least one add transmitter is a tunable transmitter.33. An optical transmission system comprising: at least one opticaltransmitter configured to transmit at least one optical signal channelvia an optical signal along an optical path; at least one opticalreceiver configured to receive at least one optical signal channel fromsaid optical path; and, an optical switching device disposed along saidoptical path including: a demultiplexer configured to demultiplex theoptical signal provided to an input port into a plurality of signalgroups, each signal group including at least one signal channel, aswitch element configurable to pass or prevent the passage of at leastone of the signal groups and variably control the optical power of thesignal groups passing through said switch element, and a multiplexerconfigured to multiplex the signal groups passed by said switch elementinto an output signal through an output port in optical communicationwith said optical receiver, wherein said optical switching deviceimparts a substantially uniform optical loss across the at least oneoptical signal channels; a distributor having an input port and aplurality of output ports and configured to distribute an optical addsignal provided at the input port into a plurality of optical signals atthe output ports, wherein at least one output port from the distributoris connected to at least one input port of the multiplexer via at leastone switch element; a combiner having a plurality of input ports and anoutput port and configured to combine optical signals provided at theinput ports into at least one optical drop signal at the output port,wherein at least one input port to the combiner is connected to at leastone output port from the demultiplexer via at least one switch element.34. The optical transmission system of claim 33, wherein the distributoris a demultiplexer.
 35. The optical transmission system of claim 33,wherein at least one output port of the distributor is connected to atleast one input port of the combiner.
 36. The optical transmissionsystem of claim 35, further comprising at least one switch elementconnected between at least one output port of the distributor and atleast one input port of the combiner.
 37. The optical transmissionsystem of claim 33, wherein at least one add transmitter and at leastone drop receiver are remotely located from the switching device. 38.The optical transmission system of claim 34, wherein the Input port ofthe distributor and the output port of the combiner are connected to alocal optical transmission system.
 39. The optical transmission systemof claim 38, wherein the distributor and the combiner form a portion ofan add/drop multiplexer in the local optical transmission system. 40.The optical transmission system of clam 38, wherein the at least oneoutput port of the distributor connected to the at least one input portof the combiner forms a through path in the local optical transmissionsystem.
 41. The optical transmission system of claim 38, furthercomprising a transmitter connected to the input port of the distributor,wherein the transmitter is located in the local optical transmissionsystem and is remote from the switching device.
 42. The opticaltransmission system of claim 41, further comprising a receiver connectedto the output port of the combiner, wherein the receiver is located inthe local optical transmission system and is remote from the switchingdevice.
 43. An optical switching device comprising: a demultiplexerincluding an input port and a plurality of output ports, wherein thedemultiplexer is configured to demultiplex an, optical signal providedto the input port into a plurality of signal groups at the output ports,each signal group including at least one signal channel, a switchelement configurable to pass or prevent the passage of at least one ofthe signal groups and variably control optical power of the signalgroups passing through said switch element, and a multiplexer configuredto multiplex the signal groups passed by said switch element into anoutput signal through an output port, wherein said optical switchingdevice imparts a substantially uniform optical loss across the at leastone optical signal channels; a distributor connected at the input portof the demultiplexer; at least one drop receiver connected to an outputport of the distributor; a combiner connected at the output port of themultiplexer; and at least one add transmitter connect to an input portof the combiner.